- Silence -Wheader-hygiene warning on Clang. Between fx_agg_driver.cpp
and fxjs_v8.h there are hundreds or thousands of things that depend on
"using namespace", so it is best to just suppress this warning than
fix it.
- gzguts.h: Include unistd.h on all non-Windows platforms, not just
Apple platforms. Fixes implicit include warnings on Linux.
- Added parens to silence a warning about mixing && and ||.
- Removed a check if an enum is negative. Enums can be unsigned,
generating an always-false warning. The check isn't necessary.
- Removed some statements that are just names of variables that do
nothing.